Catalyst for elimination of pollutants in auto-exhaust gas

ABSTRACT

There is disclosed a catalyst composition comprising a support material upon which is deposited an alkaline earth metal oxide, chrominum trioxide, and copper oxide in proportions which achieve desirable conversion temperatures and catalyst stability for use in a catalytic muffler for auto-exhaust gases. The method of catalyst preparation is also disclosed.

United States Patent Whitman et al.

[ Sept. 23, 1975 CATALYST FOR ELIMINATION OF POLLUTANTS IN AUTO-EXHAUSTGAS Inventors: Robert Henry Whitman, Stamford,

Conn.; Louis Leonard Lento, Jr., Upper Saddle River, NJ,

Assignee: American Cyanamid Company,

Stamford, Conn.

Filed: Jan. 25, 1974 Appl. No.: 436,828

Related US. Application Data Division of Ser. No 254,593, May 18, 1972,Pat. No. 3,819,534.

US. Cl. 423/2l3.2 Int. Cl. B01D 53/34 Field of Search 423/2132; 55/DIG.3O

[56] References Cited UNITED STATES PATENTS 3,257,163 6/1966 Stiles423/2l3.2 3,444,099 5/1969 Taylor 3,787,322 1/1974 Roberstein 423/2132 XPrimary ExaminerG. O. Peters Attorney, Agent, or Firm-William J. van Loo7 Claims, No Drawings CATALYST FOR ELIMINATION OF POLLUTANTS INAUTO-EXHAUST GAS This invention is a divisional of application Scr. No.254,593, filed May 18, 1972, now US. Pat. 1N0. 3,819,534, issued June25, 1974.

This invention relates to novel oxidation .catalysts and to methods bywhich said catalysts are obtained. The invention'also relates toa'process for using the catalysts to oxidize hydrocarbon combustionexhaust gases and thereby reduce environmental pollution hazards. i

The exhaust gases bon fuels such as gasoline, diesel fuel, and the likein internal combustion engines contain mixtures of carbon monoxide andvarious hydrocarbons, tbth satu-' rated and unsaturated, nitrogen andother constituents- These mixtures are both poisonous and obnoxious.

In addition to the known hazards resulting from the inhalation ofcombustion exhaust gases of hydrocarbon fuels, such gases have, ofcomparatively recent times, been identified with smog formation and, toa lesser extent, with various forms of cancer.

The hazards and nuisance created by hydrocarbon combustion exhaustgases, particularly olefins and carbon monoxide, from internalcombustion engines have, over the years, resulted in a number ofprocesses, catalysts, and apparatus whereby the reduction or eliminationof the harmful components of these gases has been the primary object.

A relatively common device employed forthis purpose hasbeen what isreferred to as a catalytic muffler which normally refers to a devicewhich is to be substituted into the exhaust line of an internalcombustion engine in lieu of an ordinary muffler. This has been aparticularly preferred area of activity in view of the I fact that thecost of such a device is reduced by the cost of a conventional muffiernormally employed, and by other obvious advantages. Such devices areinserted ployed therein. Prior catalysts'generally have a 'comvparatively short activity life and the difficulties of recharging thedevice with fresh catalyst have hampered from the combustion ofhydrocarvproperty requirements must be satisfied. First, of

catalyst particles-also permits more freedom of movement of individualparticles which, in turn,-under the influence of mechanical stock suchas normally is associated with a moving automobile powered by aninternal combustion engine, can cause severe attrition of the catalystparticles thus resulting in production of fines, which provides anincreased amount of bypasses or channels. v

lnconsidering a catalyst composition for the specific application incatalytic mufflers, therefore, a number of course, the catalystcomposition must be-highly active in the oxidation of carbon monoxideandolefin hydrocarbons in the exhaust fumes. This activity must beobtained without the need for large capacity air induction devices to beused in conjunction with the catalytic muffler. Next, the temperaturesat which the catalyst is warm-up of a given volume of catalyst tobeachieved.

their adoption. Prior catalysts also have the deficiency.

of requiring high temperature operation before effective oxidation isobtained, thus requiring long warm-up time before control of thehazardous and obnoxious content of exhaust gases is effective. The priorart catalysts are generally poor in their ability to oxidize carbonmonoxide unless large capacity air induction devices.

alyst particles, in turn, provides bypasses, or channels,

through which exhaust gases can escape without under going a desiredamount of oxidation. Shrinkage of the 'In accordance with the presentinvention, there isprovided a catalyst composition which comprises asupport material upon which is deposited from about 1.5% to about 10% ofan alkaline earth metal expressed as its oxide weight, from about 1% toabout 12% of copper expressed as its oxide weight, and from about 0.5%to about 40% of chromium expressed as its oxide weight, said weightsbeing based-on the weight of said support material, so as to provide 'acatalyst composition which effects about 50% conversion of carbonmonoxide at a temperature of about 550F. or less, effects about 50%conversion of hydrocarbons at a temperature of about 650"F. or'less, andexhibits shrinkage of less under the conditions of .use.

There is also provided a process for preparing'the I catalystcomposition which comprises treating a sup-' port material with fromabout 1.5% to about 10% of mi alkaline earth metal compound, expressedas its oxide weight, calcining the'treated support material at a ternperature of about 1800Fto convert the alkalineearth metal compound tothe oxide and stabilize the support material, treating the stabilizedsupport with from about 0.5% to about 40% of a chromium compound,

expressed as its oxide weight, and from about 1.0% to I about 12% of acopper compound, expressed as its oxide weight, said weight percentagesbeing based on the weight of said support material, and again calciningthe thus-treated support material to convert the added metal compoundsto their oxides.

There is also provided a process for oxidizing carbon monoxideandhydrocarbons which comprises contacting said gases with the novelcatalyst composition.

As the support materialused in the catalyst composition of the presentinvention may be employed various forms of alumina, silica. magnesia,zirconia, and mixtures thereof such as alumina/silica. A particularlypreferred support material is-gamma alumina and intermediates which leadto this form of alumina. The particular support material is prepared insuitable form in acabout 14% or' cordance with conventional procedures.By suitable form is meant that form which is normally prepared fortreatment with promoters. The form may be that of extrudatcs, preparedby mulling or other techniques, of spheres, coarse grains, tablets, andthe like, all prepared by conventional techniques. It is generallypreferred, however, to employ extrudates, since support material in thisform is better suited for use in catalytic mufflers, a preferred usageof the catalyst composition. The preformed support should be calcinedprior to use in the present invention.

It is frequently advantageous to employ an alumina which is silicastabilized. By the expression silica stabilized" is meant an aluminawhich contains from about 2 to'about 10% silica. A preferred compositioncontains about 5% silica. The use of this type of alumina as basematerial produces a more stable catalyst which is markedly moreresistant to shrinkage upon drying and calcining. However, alumina aloneprovides an outstanding base or support material.

According to one procedure, a suitable alumina for catalytic purposesmay be prepared by precipitation from a solution of alkali metalaluminate, such as sodium and potassium'aluminate, as a result of theaddition of an acid, such as sulfuric acid, or of aluminum sulfate, suchas in commercial alum. When such hydrated alumina is spray dried, it isusually characterized by a low apparent bulk density usually within therange of about 0.2 to about 0.3 cc. per gm. Such a base material is wellsuited for use in the present invention, the details of preparationbeing described in US. Pat. No. 2,657,l l5. The base material preparedas described above; is mulled and extruded, then dried in an oven andfinally calcined at l l()()F. prior to use in the present invention.

For. this invention, in order to have the required activity, shrinkageresistance. stability to steaming, permeability to gas flow (to preventexcessive back pressure), catalytic activity for the conversion ofhydrocar hens and carbon monoxide, low ingition temperature, and othercited requirements, it is preferred that the base material have a porevolume of about 0.3 to about 1.4 cc. per gm. and a surface area ofbetween 2U035() square meters per gram. It is also desirable that theparticle size have a minimum diameter of at least l/20 of an inch toabout [/5 of an inch and preferably about l/l0 of an inch.

The selected support material, preferably a calcined alumina extrudate,is first treated with a sufficient quantity of an alkaline earth metalcompound to provide' from about .l.5/( to about HIV: alkaline earthmetal oxide based on the weight of the calcined support material so asto stabilize said support against excessive shrinkage under conditionsof use. Alkaline earth metals suitable for use include calcium, barium,strontium and mixtures thereof, with calcium being preferred. Suitablesalts are those which may be impregnated uniformly on the supportmaterial. Aqueous solutions are greatly preferred and, accordingly,water-soluble salts such as calcium nitrate and the like areparticularly preferred. The solution of alkaline earth metal compounduscd to impregnate the support material will be of proper concentrationand amount to deposit the required amount of alkaline earth metalexpressed as its oxide thereon.

After the support material has been impregnated with the alkaline earthmetal compound by any of the conventional impregnation techniques, it isoven dried at a temperature of about 250F. for about an hour and thencalcined at I800F. for at least one hour to convert the alkaline earthmetal compound to its oxide and stabilize the support material. Thetemperature of l80()F. must be reached to obtain the desired stability,and lower temperatures do not produce acceptable results. Highertemperatures are not necessary and increase process costs without addedbenefits.

The primary purpose of the use of alkaline earth metal oxide in thefinal catalyst composition is to stabilize the same against excessiveshrinkage during use. Depending upon the amounts of promoter metals tobe applied subsequently, the range of about 1.5% to about 10% calcium,expressed as its oxide weight, based on the weight of the calcinedsupport material, will gener ally provide catalyst compositions thathave a shrinkage of 14% or less under conditions of use. High usages ofcopper as promoter generally causes increased shrinkage under conditionsof use and necessitates usages of alkaline earth metal oxide at theupper level of the specified range. High usages of chromium as pro motergenerally reduces shrinkage tendencies of the support and enables usagesof alkaline earth metal oxide at the lower level of the specified range.Excessive usage of alkaline earth metal oxide stabilizer should beavoided since such usage tends to reduce effectiveness of the finalcatalyst composition. Preferred usage of alkaline earth metal oxide isfrom about 2.0% to 5.0%, more preferably about 3.0%, by weight based onthe weight of the calcined support material.

The first of the promoter metals required in the catalyst composition ofthe present invention is chromium in the form of its oxide. Chromicoxide may be employed in the broad range of from about 0.5% to about40%, by weight, based on the weight of the calcined support material. inthose applications where the catalyst composition is to be used in acatalytic muffler, however, it is generally preferred to limit the upperlevel of usage to about 14%, by weight, based on the weight of thecalcined support material in order to obtain a final composition ofdensity sufficiently low to provide rapid warm-up of the catalyticmuffler bed.

The specific amount of chromic oxide that is used in any given instancewill vary depending upon the amounts of copper oxide and alkaline earthmetal oxides present. ln any event, the amount of chromic oxide usedwill be properly selected from the range specified so as to giveconversion temperatures for carbon monoxide and hydrocarbons of about550F. or less and 650F. or lcss,'respcctively, said temperatures beingthose at which about 50% conversion of the exhaust gas componentsoccurs.

The other of the promoter metals required in the catalyst composition ofthe present invention is copper in the form of its oxide. Copper oxidemay be employed in the broad range of about l.07( to about 12.0%, byweight based on the weight of the calcined support material. Thespecific amount of copper oxide that is used in any given instance willalso vary, depending upon the amounts of chromic oxide and alkalineearth metal oxide present. ln any event, the amount of copper oxide usedwill be properly selected from the range s ecified so as to provideconversion temperatures as indicated. The amount of copper oxide thatmay be employed is considerably more limited than that of chromic oxidedue to adverse effects on catalyst shrinkage.

as indicated above. Accordingly, in addition to selecting an amount ofcopper oxide that will provide the desired conversion temperatures, itis also necessary to make such selection in keeping with the shrinkagerequirements of the final catalyst composition under conditions of use.Preferably the amount of copper oxide is in the range of about 2% to 9%.

In depositing the chromium and copper promoters on the alkaline earthmetal oxide modified support material, conventional impregnationprocedures are employed. While it is possible to deposit the promotersin combination or in various orders it is greatly preferred to depositchromium promoter material first and then, in a subsequent step, todeposit copper promoter material. Various techniques such as spraying,pore saturation, and the like may be used. It is generally preferred toemploy an aqueous medium to effect impregnation and, accordingly, it ispreferred to employ watersoluble compounds of the promoter materials,which are converted upon calcination to the oxides. In carrying outimpregnation of the promoter metals in a sequential manner, it isnecessary, of course, to ensure that the subsequent impregnation doesnot remove or interfere with the prior' impregnation. Thus, insequential impregnation of the promoter metals, it is greatly preferredto employ a so-called quick calcination" following impregnation of theindividual promoter metal compounds so as to prevent removal bydissolution in the subsequently applied solution of the secondindividual promoter. By quick calcination as that term is employedherein is meant a calcination at a temperature of I100F. for about anhour without an intermediate drying step. Normally, the impregnatedsupport materials are first dried, then subjected to calcination. In thepresent invention, it has been found that by introducing the wetimpregnated support materials directly into the calciner andquick-calcining at l 100F. for an hour, better activity results.

In a preferred embodiment of the invention, the calcined formed supportmaterial is impregnated with an aqueous solution of a calcium salt suchas calcium nitrate, calcium acetate, calcium nitratemonohydrate, andthelike so as to obtain the desired level of calcium. The impregnatedsupport is then dried at 250F. in an oven for approximately one hour andthen calcined at I800F. for at least one hour to obtain calcium oxidestabilized support material. The calcium oxide stabilized support isthen impregnated with an aqueous solution of a chromium compound such aschromic acid so as to obtain the desired level of chromium. The wetimpregnated support is quick-calcined at l 100F. for one hour. Thechromium promoted calcium oxide stabilized support is then impregnatedwith an aqueous solution of a copper salt such as copper nitratetrihydrate and then subjected to quick-calcining at ll00F. for

one hour.

Of the great number of characteristics which are desired in automobileexhaust gas conversion catalysts, at least three are consideredessential and have criteria defined and recognized. The first isshrinkage, that is, it is desirable to keep the catalyst particle fromshrinking under conditions of use. It is considered satisfactory if acatalyst shrinks less than about 14% and more desirable if the catalystshrinks less than of its original size. The second and third criteriarelate to the hydroearbon and carbon monoxide 50% conversiontemperatures. These temperatures are defined as those at which 50%conversion of hydrocarbon and carbon monoxide in the exhaust gas iseffected. It is, of course, desirable to have as low a 50% conversiontemperature as possible. Thus, it is desirable that the hydrocarbon 50%conversion temperature be less than about 650F. and more preferably lessthan about 600F. Likewise it is desirable that the carbon monoxide 50%conversion temperature be less than 550F. and more preferably less than500F. Of course it is possible to obtain even lower 50% conversiontcmperatures, such as 500F. for hydrocarbon and 400F. for carbonmonoxide, and such temperatures should be obtained, if at all possible.

In all of the catalyst compositions of this invention, the catalyst'ismade up of oxides of copper, chromium, and calcium, with the balancebeing support material, preferably alumina. The particular proportionsof promoter metal oxides and stabilizing alkaline earth metal oxide arein the range that fulfill the criteria specified and, therefore,eminently qualify the catalyst compositions of the present invention foruse in catalytic mufflers.

As compared with prior art catalysts suitable for use in the oxidationof exhaust gases, the catalyst composition of the instant invention hasseveral marked advantages. It results in a more effective oxidation ofharmful olefins, including ethylene and propylene, as well as carbonmonoxide. When a catalyst composition of this invention is used in acatalytic muffler associated with an automobile exhaust system, thepresence of copper results in a substantially faster warm-up of thecatalyst bed in comparison with those containing prior art catalysts.Faster warmup of the catalyst bed results principally from oxidation ofhydrocarbons and carbon monoxide by the copper oxide which performseffectively at low temperature. This oxidation at low temperatureresults in a rapid release of heat which aids in warming up the entirecatalyst bed. The resulting faster warm-up of the entire bed enablestotal conversion to be achieved at short engine warm-up times andreduces times to total conversion of the polluting hydrocarbons andcarbon monoxide. Thus, instead of relying solely on engine warmup toeffect the higher temperatures necessary for total conversion, thecatalyst composition of the present invention, by initially effectingpartial conversion at low temperature, greatly accelerates warm-up ofthe catalyst bed by supplying the heat from the initial partialconversions. In addition to the substantial reduction in warm-up time ofthe catalyst bed, the catalyst bed at the high operating temperaturesreached is more effective for olefin conversion. The catalystcomposition also enables substantial conversion' of carbon monoxide tobe achieved without the requirement for large capacity air inductiondevices. The catalyst composition also maintains good activity over longtime periods of use and has high shrinkage resistance.

The invention is more fully illustrated by the examples which follow,wherein all parts and percentages are by weight unless otherwisespecifically designated.

EXAMPLE 1 Five pounds of 1/10 inch diameter alumina-silica extrudates,which were prepared by mulling an aluminasilica powder with water andammonia and extruding the mulled mixture, were oven-dried at 250F. andthen calcined at 1lOOF. The calcined extrudates were impregnated bysprayingusing 2.47 liters of an aqueous Solution containing 0.65 poundof calcium nitrate tetrahydrate. Theimpregnated extrudates were thenoven dried again at 250F. for one hour and then calcined at 1800F. forthree hours to obtain calcium oxide stabilized extrudates containing anominal 3%. by weight. based on the weight of the original calcinedextrudates. of calcium oxide on the 95:5 aluminazsilica extrudates.

A portion. l 13.5 grams of the calcium oxide stabilized extrudatesprepared above were spray impregnated with 101 cc. of an aqueoussolution containing 5.45 grams of chromic acid (CrO The impregnatedextrudates were quick-calcined at l 100F. for one hour to produce anominal 3.5% of chromic oxide on the calcium oxide stabilizedextrudates.

The calcium oxide stabilized extrudates also containing the chromicoxide as prepared above were next spray-impregnated with 94 cc. of anaqueous solution containing 22.5 grams of copper nitrate trihydrate andagain quickcalcined to provide a nominal 5.3% of copper oxide.

Formal analyses of the final composition obtained above indicated theactual content to be 5.25% copper oxide. 2.83% chromic oxide. 2.62%calcium oxide and 89.3% aluminasilica support.

Into a reaction tube were placed 12.5 cubic centimeters of catalystcomposition prepared as above and dried to remove any absorbed water.The catalyst com position was heated to 593C. the furnace turned off.and a test exhaust gas mixture introduced. The test cxhaust gas mixturewhich was used to simulate raw exhaust gas was a mixture of 1% carbonmonoxide, 250 parts per million of propylene. 2.5% oxygen. and thebalance nitrogen. This test exhaust gas mixture passed through a watersaturator set at 46C. prior to introduction. and a space velocity of13.200 per hour was maintained. As the catalyst composition cooled.samples of the exit gas were taken and from these the present conversionwas determined. A plot of percent conversion versus temperature was madeand from the resulting graph. the temperature corresponding to 50%conversion was obtained for both carbon monoxide and prop ylene. Thesetemperatures were 473F. and 478F.. respectively. The percentageshrinkage of the catalyst under conditions of use was 1 1.6%.

EXAMPLE 2 Example 1 was repeated in every material detail except thatthe impregnations were varied so as to obtain a final composition whichcontained 5.75% copper oxide. 3.5% chromic oxide. and 3.0% calcium. Whentested as in Example 1. the 50% conversion temperatures were 527F. forhydrocarbons and 470F. for carbon monoxide. The percentage shrinkage ofthe catalyst was 13.6%.

EXAMPLE 3 A sample of exhaust gas catalyst comprising 51% by weightot'cupric oxide and 1 1.3% by weight ofchromic oxide as promoters on a2.1% calcium oxide stabilized base comprising 95% alumina and silica wasprepared according to the procedure described for Example 1. When testedas in Example 1. the 50% conversion temperatures were 581F. forhydrocarbons and 500F. for carbon monoxide. The percentage shrinkage ofthe catalyst was 6.0%.

EXAMPLE 4 A sample of exhaust gas catalyst comprising 1 1.7% by weightof cupric oxide and 1 1.0% by weight of chromic oxide promoters on a1.1% calcium oxide stabilized base comprising alumina and 5% silica wasprepared following the general procedure of Example 1. When tested as inExample 1. the 50% conversion temperatures were 5 50F. for hydrocarbonsand 478F. for carbon monoxide. The percentage shrinkage of the catalystwas 10.4%.

EXAMPLE 5 A sample of exhaust gas catalyst comprising 3.3% by weight ofcupric oxide and 3.0% by weight of chromic oxide as promoters on 2.7%calcium oxide stabilized base comprising 95% alumina and 5% silica wasprepared following the general procedure of Example 1. When tested as inExample 1, the 50% conversion temperatures were 577F. for hydrocarbonsand 563F. for carbon monoxide. The percentage shrinkage of the catalystwas 9.8%.

EXAMPLE 6 A sample of exhaust gas catalyst comprising 2.2% by weight ofcupric oxide and 1.6% by weight of chromic oxide as promoters on 1.7%calcium oxide stabilized base comprising 95% alumina and 5% silica wasprepared following the general proccdure of Example 1. When tested as inExample 1. the 50% conversion temperatures were 520F. for hydrocarbonsand 527F. for carbon monoxide. The percentage shrinkage of the samplewas 6.6%.

EXAMPLE 7 A sample of exhaust gas catalyst comprising 5.0% by weight ofeupric oxide and 0.6% by weight of chromic oxide as promoters on 1.8%calcium oxide stabilized base comprising 95% alumina and 5% silica wasprepared according to the general procedure of Example 1. When tested asin Example 1. the 50% conversion temperatures were 507F. forhydrocarbons and 496F. for carbon monoxide. The percentage shrinkage ofthe catalyst was 14.4%.

We claim: I

-l. A process for oxidizing carbon monoxide and hydrocarbons in exhaustgases emanating from internal combustion engines which comprisescontacting said gases with a catalyst composition comprising a calcinedsupport material selected from alumina. silica. magnesia. zirconia. andmixtures thereof containing deposited on said support from about 1.5% toabout 10% of an alkaline earth metal oxide selected from calcium.barium. strontium. and mixtures thereof. from about 0.5 to about 40% ofchromic oxide. and from about 1% to about 12% of copper oxide. saidpercentages being by weight based on the weight of the calcined supportand said catalyst composition having a 50% conversion temperature forhydrocarbons of about 650F. or less. a 50% conversion temperature forcarbon monoxide of about 550F. or less. and a shrinkage of 14% or lessunder conditions of use.

2. The process of claim 1 wherein said support is alumina/silica 5/5.

3. The process of claim 2 wherein said alkaline earth metal oxide iscalcium oxide.

9 4'. The process of claim 3 wherein said calcium oxide present at 5.25%and chromic oxide is present at is present in the range of about 2% toabout 5%.

5. The process of claim 4 wherein chromium oxide is present in the rangeof about 1% to 14% and copper oxide is present in the range of about 2%to 9%.

6. The process of claim 4 wherein copper oxide is 7. The process ofclaim 4 wherein copper oxide is 5 present at 5.75% and chromic oxide ispresent at 3.5%.

1. A PROCESS FOR OXIDIZING CARBON MONOXIDE AND HYDROCARBONS IN EXHAUSTGASES EMANATING FROM INTERNAL COMBUSTION ENGINES WHICH COMPRISESCONTACTING SAID GASES WITH A CATALYST COMPOSITION COMPRISING A CALCINEDSUPPORT MATERIAL SELECTED FROM ALUMINA, SILICA, MAGNESIA, ZIRCONIA, ANDMIXTURES THEREOF CONTAINING DEPOSITED ON SAID SUPPORT FROM ABOUT 1.5% TOABOUT 10% OF AN ALKALINE EARTH METAL OXIDE SELECTED FROM CALCIUM,BARIUM, STRONTIUM, AND MIXTURES THEREOF, FROM ABOUT 0.5 TO ABOUT 40% OFCHROMIC OXIDE, AND FROM ABOUT 1% TO ABOUT 12% OF COPPER OXIDE, SAIDPERCENTAGES BEING BY WEIGHT BASED ON THE WEIGHT OF THE CALCINED SUPPORTAND SAID CATALYST COMPOSITION HAVING A 50% CONVERSION TEMPERATURE FORHYDROCARBONS OF ABOUT 650*F. OR LESS, A 50% CONVERSION TEMPERATURE FORCARBON MONOXIDE OF ABOUT 550*F. OR LESS, AND A SHRINKAGE OF 14% OR LESSUNDER CONDITIONS OF USE.
 2. The process of claim 1 wherein said supportis alumina/silica 95/5.
 3. The process of claim 2 wherein said alkalineearth metal oxide is calcium oxide.
 4. The process of claim 3 whereinsaid calcium oxide is present in the range of about 2% to about 5%. 5.The process of claim 4 wherein chromium oxide is present in the range ofabout 1% to 14% and copper oxide is present in the range of about 2% to9%.
 6. The process of claim 4 wherein copper oxide is present at 5.25%and chromic oxide is present at 2.83%.
 7. The process of claim 4 whereincopper oxide is present at 5.75% and chromic oxide is present at 3.5%.